Explanation:

• The sentence is in the past tense ("I didn't know Rahul was in the hospital"), indicating that the action of not knowing Rahul's situation occurred before the action of potentially visiting him.
• Therefore, to convey the correct sequence of events, the past perfect tense "had known" should be used to describe the action of not knowing Rahul's situation in the past.
• The past perfect tense is paired with the present perfect tense "have visited" to indicate a completed action that occurred after the action of not knowing.
• Thus, the completed sentence would be: "I didn't know Rahul was in the hospital. If I had known, I would have visited him."